Ultrasonic transmitting means and method of producing same



June 15,1965 R. G. GOLDMAN ErAL 3,189,767

ULTRASONIC TRANSMITTIYNG MEANS AND METHOD OF PRODUCING SAME FiledA Jan28 1963 l l a l l l l @ISG l@ I L United States Patent O f `ceULTRASUNIC TRANSMIT'IING MEANS AND METHOD F PRODUCING SAME Richard G.Goldman and Wiiliam R. Marklein, Schenectady, NSY., assignors to GeneralElectric Company, a corporation of New York Filed Jan. 28, 1963, Ser.No. 254,328

9 Claims. (Cl. 310--8.2)

This invention relates to an article of manufacture for transmittingultrasonic energy and method of producing same. The article isparticularly applicable as a matchying layer or backing member for anelectro-acoustical transducer.

In the generation and reception of ultrasonic waves for many purposes,but particularly in electro-acoustical or piezoelectric ultrasonictransducers yfor flaw detection purposes, it is desirable that theultrasonic pulse vbe short in order to achieve good definition and todetect flaws near `the surface of the object under test. Despite theapplication of short electrical pulses to the transducer, short,Well-defined ultrasonic pulses have not been realized due tooscillations or ringing in the transducer itself. To dampen this ringingand to .reduce the reflections to the transducer from its interfaceswith other mediums, transducer backings and matching layers have beendeveloped.

An ideal matching 4layer permits a maximum transmission of ultrasonicenergy to the object under test with a minimum of interface .reflectionsby providing an impedance match with the transducer interface and withthe inter-face of the object under test. The reduction of reflections aswell as the improved transmission increase the damping of the transduceroscillations. An ideal backing likewise matches the transducer impedanceexactly and :additionally provides maximum Adamping through highabsorption of energy transmitted from the rear face of the transducer.

In the past, it has not been possible to find a single material whichprovides the above-mentioned ideal characteristics of matching layers orbackings for commonly used transducers such .as quartz or ceramicpiezoelectric while presenting no discontinuities of acoustic impedance.Consequently, various expedients have been adopted in attempts tofulli-ll the requirements of absorption and impedance matching. 1n thecase of matching layers, laminations of different materials havingaverage impedance matches, various filling substances, etc. have beentried. Likewise, transducer `backings utilizing multiple, specificallydimensioned, layers of different substances, `some of which provide animpedance match and others yof whic-h have a high absorption, have beencombined in multiple layer-s.

Furthermore, the -methods employed heretofore in .the production ofmatching layers and backings have precluded the control necessary toachieve a `device having the desired characteristics of accurateimpedance matching, absorption and freedom from discontinuity. This lackof control is eremplied by devices utilizing multiple layers of variousdimensions having mult-iple bonding `layers therebetween and by variouscompositions comprising substances having particles, fibers, etc. mixedtherein to produce varying densities.

My invention overcomes the failures of .the prior art by providingbackings and matching layers having accurately controlled acousticalimpedances and freedom from discontinuities either in physical structureor in acoustical impedance. Also, the process used is capable ofproducing transducer backings having high absorption.

Accordingly, it is an object of this invention to provide improvedultrasonic `energy transmitting means free from discontinuity ofacoustic impedance.

3,189,767 Patented June I5, 1965 It is another object of this inventionto provide a matching layer of ceramic mate-rial for transmittingultrasonic energy from a first medium to a second medium in which theceramic material provides a matching impedance at the interface with thefirst medium, and a second matching impedance at the interface of saidsecond medium, and is free from discontinuity of acoustic impedance.

It is a farther object of this invention to provide a transducer backingpermitting a maximum transmission of ultrasonic energy from thetransducer to the backing and having high absorption.

It is yet another object of this invention to provide an improved methodof producing a ceramic block for transmitting ultrasonic energy.

Brieiy stated, in accordance with my invention, an ultrasonictransmitting means having the ideal characteristics outlined above isobtained by a gradient tiring process. The gradient firing is applied toa block of ceramic material which has been previously isostatical'lypressed and pre-fired, in a manner described more lspecificallyhereinafter. One end of the block is subjected to a heat source at agreater temperature than that of the pretiring, the gradient firingbeing achieved yby thermal conductivity of the ceramic material betweenthe heat-ed end and the opposite unheated end of the block. Ceramicmaterial treated in this manner may be produced under controlledconditions of time, temperature and composition to meet varyingimpedance matching and damping requirements for ultrasonic transmittingdevices.

For a complete understanding of my invention, reference may be had tothe accompanying drawing, in which:

FIG. 1 shows, in vertical cross-section, the transmitting means of thisinvention used as a backing for a piezoelectric crystal.

FIG. la is a graph representing the gradient firing temperaturesachieved by thermal conductivity throughout the backing means of FIG. l.

FIG. 2 shows, in vertical cross-section, the transmitting means of thisinvention used as a matching layer between a piezoelectric crystal andan object under test.

FIG. 2a is a graph of the gradient firing temperature achieved bythermal conductivity through the transmitting means of FIG. 2.

FIG. 3 shows, in diagrammatic form, one method of gradient tiring inaccordance with the present invention.

The ultrasonic transmitting means of the present invention may be usedfor many purposes related to the generation and reception of ultrasonicwaves. However, for purposes of illustration, my invention is describedin this application in connection with the use of backings and matchinglayers for piezoelectric crystals as used, for instance, in ultrasonicflaw detection apparatus.

Referring now to the drawings, there is shown in FIG. l a piezoelectricelement 1t) which may be in the Iform of quartz or ceramic cry-stal towhich high-frequency electric oscillations may be applied to effecthigh-frequency mechanical vibrations of the crystal in a manner wellknown in the art. To damp the crystal, a backing or ceramic blockindicated generally at 15 is joined to the rear surface 11 of theelement 1) by means of a thin layer of any suitable bonding agent 13,such as, for instance, an epoxy resin.

In FIG. 2, a piezoelectric element 10 is shown having its front surface12 attached by a bonding agent 13 to a ceramic block or matching layerindicated at 20. The matching layer 20 transmits ultrasonic energybetween the element 10 and a medium or object 21 under test. Thematching layer 20 at one end 22 has an impedance which matches that ofthe element 1d, and at the opposite end 23 has an acoustic impedancewhich matches that of the object 21. The matching layer 20 may be usednot only in cooperation with piezoelectric elements but may ed be usedbetween any two media for the transmission of ultrasonic energytherebetween.

The transmitting means shown respectively in FIGS. 1 and 2 as backing i5and matching layer 20 comprises a ceramic material which has beengradiently fired in a manner illustrated in FIG. 3. The gradient firingused in accordance with my invention is applied to a ceramic materialwhich has been formed to a solid mass or block by conventional means,such as isostatic pressing, whereby powder is compressed in a flexiblemold by the uniform application of pressure through a huid in which themold is immersed. Conventional steel die pressing or extruding processesmay also be utilized. The block is then pre-fired to sufficiently bondthe ceramic material, producing a homogeneous, relatively soft,chalk-like block 3d.

The gradient ring process may be accomplished, for example, in a furnacecomprising a steel vessel 31 which is capable of being evacuated. A heatsource is provided by an annular sleeve or susceptor 32, having ahigh-frequency coil 32 Wrapped about its outer surface, the susceptorbeing mounted within vessel 31 in any suitable manner. The pre-firedblock 3i) is placed within the vessei 3l. with one end 3e resting on alarge cast iron block 3S serving as a heat sink, and the other end 36disposed within the graphite susceptor 32. Air is then evacuated fromthe Vessel and a high-frequency current passed through coil 33, heatingthe block by radiation from the susceptor 32. The firing of the block ina vacuum insures maximum density at the end 36 of the block and freedomfrom heating, by convection currents of gas in chamber 3i, of the end3d. The temperature applied to surface 36 by susceptor 32 is greaterthan the pre-tiring temperature. The opposite end 34tof block 3@ ismaintained at a relatively low temperature by means of conduction ofVheat to the sink 35, the block being thereby fired by the temperaturegradient established between the ends 34 and 35 due to the thermalconductivity of the ceramic material.

FIGS. la and 2a illustrate graphically the tiring temperature gradientwhich exists across the thickness of backing member l5 and matchinglayer 2d respectively. The temperature and resulting density of theceramic point A in FiG. 2a will be determined by the impedance desiredto match that of the object 21 under test. The end of block 29 abuttingthe object 2] may be red at a higher or lower temperature than the end22, depending on the acoustic impedance of the object 21. It will ofcourse be understood that the end desired to be fired at the highertemperature will be disposed within the heating device 32, and the lowertemperature end will abut thev heat sink 35. The temperature attained atthe lower temperature end of the block will of course depend on thematerial, size, and shape of the block being fired, the temperature atwhich the hot end is fired, the characteristics of the heat sink 35, thelength of the firing cycle, etc.

Using the ceramic materials discussed below in this gradient firingprocess, it has been found that a backing material having an acousticalimpedance matching that of the rear surface of a piezoelectric elementcan be produced, while at the same time providing high absorption andtherefore maximum damping of the ultrasonic energy transferred to thebacking. The more dense end lof backin 7 15 which has been heated to thehigher temperature, being adjacent the heating element during thegradient firing process, provides a matching impedance for the rearsurface l1 of the element 10. The end le of the backing T is less dense,having been fired to a lower temperature, and provides high absorptioncharacteristics. Furthermore, the density of the block uniformlydecreases between ends 14 and 16 and provides no discontinuity ofacoustical impedance, since a single homogeneous ceramic material isused.

Turning now to the matching layer of FlG. 2 produced by the gradientring process, the end 22, having been heated to the proper temperatureas shown in FIG. 2, provides a matching impedance for the transducerelement l?. in this case, the temperature at the end 23 of the matchinglayer 2G is controlled so as to provide an impedance match with theobject 21 under test. As in the case of the backing 15' of FIG. l, thematerial of matching layer 29 has a density uniformly varying from end22 to end 23 wmie eing free from discontinuity of acoustical impedance,which would create undesirable reflections from reaching the elementil?.

it has been found that very satisfactory results are obtained when theceramic material of the matching means is formed from a powderedrefractory oxide. Certain aluminum oxide powders sold under thetrademarks 3S-900 Alundum, by the Norton Company, and Linde A, by LindeAir Products Company, when compressed and fired in the above manner,have produced excellent backings or matching layers for ceramic andquartz piezoelectric crystals. These materials may be cornbl red withsmall amounts of other materials. The 3S- SCO Alundum comprises arelatively pure (99.9%) fused alumina having an average particle size of7 microns. The Linde A material consists of a very fine (0.3 micron) andpure (99-{%) aluminum oxide obtained by the controlled calcination of analuminum sulphate compound.

When fired in the range of l200 C. to 1800 7 C., the 3S-900 Alundumcompressed at 10,06() p.s.i. forms a ceramic body or block in which thedensity and the sonic velocity vary over a wide range with firingtemperature. At approximately 1620 C. firing temperature, the irnpedance(Z) of the ceramic formed from the 3S-9G() Alundum, which is the productof p, the density, and v, the sonic velocity, (that is, Z zpv) is foundto equal that of many of the commonly used ceramic piezoelectricelements which are formed from proprietary formulas based on bariumtitanate or lead zirconate titanate, such as that sold under the tradename Gulton HT. When tired at approximately l530 C., the impedance ofthe alumina ceramic matches that of quartz.

Improved damping results from using a transducer backing, all of whichhas been initially subjected uniformly to such temperatures. However,maximum damping is provided by initially pre-firing the entire backingat a uniform temperature of ll00 C. and then, in producing a transducerbacking from the compressed 3S-960 Alundum, gradient firing the backingat a maximum ternperature of either l620 C. or 1530 C., depending onwhether the piezoelectric element with which the backing is to be usedis of ceramic or quartz. Gf course, in the case of matching layers, thepre-tiring temperature permits different acoustical impedances to existat the respective ends of the block.

With respect to the Linde A material, it has been found that anisostatic pressing at 10,000 p.s.i., a uniform pre-tiring at 1100 C. anda gradient tiring at 1465L1 C. provide an excellent backing material forthe common piezoelectric ceramic materials. A sample produced by thisprocess was found to substantially shorten the ultrasonic pulse, toincrease damping, and to eliminate detectable'reections from thetransducer interface when used with a 2.25 megacycle barium titanatetransducer having a one-inch diameter.

When using either of the above-mentioned alumina ceramics, thepre-firing process results in a block having a chalk-like consistencywhich permits machining of the block to any desired configuration orsize prior to the gradient firing. Also the pressed material may bemachined or ground to conform to wanted dimensions prior to thepre-firing step.

Refractory om'des, including magnesium oxides and the 3 8-900 Alundumand the Linde A alumina ceramics, have a very poor heat transmittingcharacteristic. For this reason, such materials are ideal for thegradient firing technique, since the end to which the heat source isapplied may be highly heated while the opposite end of the block may bemaintained at a relatively low ternperature, thereby achieving thedesirable varying density while avoiding discontinuity of acousticalimpedance. The poor heat transmission characteristic additionallypermits backings and matching layers of minimum thickness, as the end ofthe block away from the heat source will be heated very slowly even whena thin block is used. Although the ceramic block is illustrated in FIGS.l, T2 and 3 as a rectangular prism or cylinder, the invention is not solimited but may take any suitable geometric form.

While the invention has thus been disclosed and the presently preferredembodiment described, it is not intended that the invention be limitedto the applications discussed herein. instead, many modifications willoccur to those skilled in the art which lie within the spirit and scopeof the present invention.

Having described the invention, what is claimed is:

1. For use with a piezoelectric element having two spaced surfaces, atransmitting means comprising a homogeneous ceramic block, said blockhaving one end engaging one of said surfaces, said block havinguniformly changing density from ysaid one end to the other end of saidbloei(` so that the acoustical impedance of said block varies uniformlywhereby said block is rendered free from discontinuity of acousticalimpedance.

2. For use with a piezoelectric element having two spaced surfaces, atransmitting means comprising a homogenous ceramic block of refractoryoxide, said block having one end engaging one of said surfaces, saidblock having uniformly changing density from said one end to the otherend of said block so that the acoustical impedance of said block variesuniformly whereby said block is rendered free from discontinuity ofacoustical impedance.

3. For use with a piezoelectric element having two spaced surfaces, atransmitting means as specied in claim 2 in which the refractory oxideis an alumina ceramic.

4. For use with a piezoelectric element having two spaced surfaces, abacking comprising a gradient red homogeneous ceramic block, said blockhaving one end engaging one of said surfaces and having a density whichincreases uniformly from said one other end to a maximum at the otherend so that the acoustic impedance of said block at said one end issubstantially the same as the acoustical impedance of said element andsaid acoustical impedance of said block increases uniformly from saidone end to said other end.

5. A matching layer for transmitting ultrasonic energy from a firstmedium to a second medium comprising a homogeneous ceramic block, saidblock having one end engaging said first medium, `said block having adensity at said one end providing an acoustic impedance substantiallythe same as the acoustical impedance of said rst medium and a density atthe other end providing an acoustical impedance substantially the sameas the acoustical impedance of said second medium, said block havinguniformly charging density between said ends whereby said block isrendered free from discontinuity of acoustical impedance.

6. A method for forming a backing or transmitting member for ultrasonicenergy comprising the steps of:

(a) forming a homogeneous ceramic block, and

(b) gradient firing said block by applyin7 a heat source to one endthereof to provide uniformly changing density between said one end andthe other end of said block so that the acoustical impedance of saidblock varies uniformly whereby said block is rendered free fromdiscontinuity of acoustical impedance.

7. A method of forming a backing or transmitting member for ultrasonicenergy comprising the steps of:

(a) forming a homogeneous block of refractory oxide material,

(b) pre-firing ysaid block uniformly at a rst temperature, and

(c) gradient firing said block by applying a heat source having a secondtemperature greater than said first temperature to one end of said blockto provide uniformly changing density between said one end and the otherend of said block so that the acoustical impedance of said block variesuniformly whereby said block is rendered free from discontinuity ofacoustical impedance.

S. A method of forming a backing or transmitting member for apiezoelectric element comprising the steps of (a) forming a homogeneousblock of refractory oxide material,

(b) pre-firing said block uniformly at approximately 1100 C., and

(c) gradient firing said block by contacting a heat sink member with oneend of the block and applying a heat source 'in the range of 1400 C. tol650 C. to the other end thereof to provide uniformly changing densityfrom said one end to the other end of said block so that the acousticalimpedance of said block varies uniformly whereby said fired block isrendered free from discontinuity of acoustical impedance.

9. The method of forming a piezoelectric backing or matching member inaccordance with claim 8 in which the gradient firing step is carried outin a vacuum to reduce the transfer of heat by convection.

References Cited bythe Examiner UNITED STATES PATENTS 2,430,013 11/47Hansell S10-8.2 2,707,755 5/55 Hardie et al. 310-8.2 2,972,068 2/61Howry et al. S10-8.2

MILTON O. HIRSHFIELD, Primary Examiner.

1. FOR USE WITH A PIEZOELECTRIC ELEMENT HAVING TWO SPACED SURFACES, ATRANSMITTING MEANS COMPRISING A HOMOGENEOUS CERAMIC BLOCK, SAID BLOCKHAVING ONE END ENGAGING ONE OF SAID SURFACES, SAID BLOCK HAVINGUNIFORMLY CHANGING DENSITY FROM SAID ONE END TO THE OTHER END OF SAIDBLOCK SO THAT THE ACOUSTICAL IMPEDANCE OF SAID BLOCK VARIES UNIFORMLYWHEREBY SAID BLOCK IS RENDERED FREE FROM DISCONTINUITY OF ACOUSTICALIMPEDANCE.